U.S. patent number 6,742,035 [Application Number 09/515,087] was granted by the patent office on 2004-05-25 for directory-based volume location service for a distributed file system.
This patent grant is currently assigned to Novell, Inc.. Invention is credited to Stephen R. Carter, Delos C. Jensen, Stephen G. Toner, Edward R. Zayas.
United States Patent |
6,742,035 |
Zayas , et al. |
May 25, 2004 |
**Please see images for:
( Certificate of Correction ) ** |
Directory-based volume location service for a distributed file
system
Abstract
In a hierarchically organized distributed file system, resources
belong to the containers in which they are located. These resources
do not have to be co-located with the, containers in which they are
stored, but can be distributed over the file system. To locate a
desired resource, the unique name of the resource is determined.
Moving upward through the containers in the hierarchically
organized distributed file system, the lowest container with a
location service is determined. The location service is queried for
known instances of the resource. Any instance of the resource
returned from the location service can then be utilized. Semantic
contexts can also be applied to the network to control access or
usage of both the location services and resource instances. The use
of semantic contexts allows for improvements in network usage,
security, resource allocation, and the like.
Inventors: |
Zayas; Edward R. (Salem,
UT), Toner; Stephen G. (Sundance, UT), Jensen; Delos
C. (Orem, UT), Carter; Stephen R. (Provo, UT) |
Assignee: |
Novell, Inc. (Provo,
UT)
|
Family
ID: |
32313225 |
Appl.
No.: |
09/515,087 |
Filed: |
February 28, 2000 |
Current U.S.
Class: |
709/226;
707/E17.01; 707/999.01; 707/999.004 |
Current CPC
Class: |
H04L
67/16 (20130101); H04L 69/329 (20130101); G06F
16/10 (20190101); Y10S 707/99934 (20130101); Y10S
707/99933 (20130101) |
Current International
Class: |
H04L
29/08 (20060101); G06F 015/173 (); G06F
017/30 () |
Field of
Search: |
;707/10,501.1,3,100,4,200 ;709/224,225,226,203,219 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Schnatz, R.; Woznick, B., Bono, G., Burke, E., Geyer, S., Hoffman,
M., MacGregor, W., Sands, R., Thomas, R. and Toner, S., Cronus, A
Distributed Operating System: Interim Technical Report No. 2, Bolt
Beranek and Newman, Inc., Report No. 5261, Feb. 1983 pp. 12-17,
31-33. .
Zayas, Edward R., AFS-3 Programmer's Reference:BOS Server
Interface, Chapter 1, pp. 1-2, 65-70, Version 1.0, FS-00-D161,
Transarc Corporation, Aug. 28, 1991. .
Zayas, Edward R., AFS-3 Programmer's Reference: Volume
Server/Volume Location Server, Interface, Chapter 1, pp. 1-2,
Chapter 2, pp. 4-5, 14, 26, Version 1.0, FS-00-D165, Transarc
Corporation, Aug. 29, 1991. .
Zayas, Edward R., AFS-3 Programmer's Reference: Architectural
Overview, pp. 17-18, Version 1.0, FS-00-D160, Transarc Corporation,
Sep. 2, 1991. .
Microsoft Professional Developer's Conference 97 slides, San Diego,
CA..
|
Primary Examiner: Wiley; David
Assistant Examiner: Delgado; Michael
Attorney, Agent or Firm: Marger Johnson & McCollom
PC
Claims
We claim:
1. A method for using a first,location service to locate a
resource, the method comprising: finding a set of location services
for a management context containing the resource, wherein the
management context includes a set of nesting containers containing
the resource and the set of location services includes at least the
first location service, each location service in the set of
location services capable of operating independently of the others;
querying the first location service for the locations of at least
one instance of the resource using a human-readable name for the
resource and the management context; receiving a set of resource
instances, wherein the set of resource instances includes at least
a first resource instance, and each resource instance is a
instantiation of the resource; and utilizing the first resource
instance.
2. A method according to claim 1 wherein finding a set of location
services includes: organizing the computer system into containers
to establish the management context; finding a lowest container
within the management context that contains the resource; and
obtaining the set of location services from the lowest
container.
3. A method according to claim 1 wherein querying the first
location service includes querying a second location service for
the location of the resource instances if the first location
service does not respond to a query.
4. A method according to claim 1 wherein utilizing the first
resource instance includes utilizing a second resource instance if
the first resource instance does not respond to a request to
utilize the first resource instance.
5. A method according to claim 1 further including informing each
location service in the set of location services when a resource
instance is added or removed from the computer system.
6. A method according to claim 1 wherein: the method further
comprises applying a semantic context to the set of resource
instances received from the first location service; and utilizing
the first resource instance includes selecting the first resource
instance based on the semantic context applied to the set of
resource instances.
7. A method according to claim 1 wherein: the method further
comprises applying a semantic context to the set of location
services for the management context containing the resource; and
querying the first location service includes selecting the first
location service based on the semantic context applied to the set
of location services.
8. A computer-readable medium containing a program implementing an
algorithm for using a first location service to locate a resource,
the program comprising: finding software to find a set of location
services for a management context containing the resource, wherein
the management context includes a set of nesting containers
containing the resource and the set of location services includes
at least the first location service, each location service in the
set of location services capable of operating independently of the
others; query software to query the first location service for the
locations of at least one instance of the resource using a
human-readable name for the resource within the management context;
reception software to receive a set of resource instances, wherein
the set of resource instances includes at least a first resource
instance, and each resource instance is an instantiation of the
resource; and utilization software to utilize the first resource
instance.
9. A computer-readable medium containing a program according to
claim 8 wherein the finding software includes: container-finding
software to find a lowest container within the management context
that contains the resource; and obtaining software to obtain the
set of location services from the lowest container.
10. A computer-readable medium containing a program according to
claim 8 wherein the query software includes second-query software
to query a second location service for the locations of the
resource instances if the first location service does not respond
to a query.
11. A computer-readable medium containing a program according to
claim 8 wherein the utilization software includes
second-utilization software to utilize a second resource instance
if the first resource instance does not respond to a request to
utilize the first resource instance.
12. A computer-readable medium containing a program according to
claim 8 further including informing software to inform each
location service in the set of location services when an instance
of the resource is added or removed from the computer system.
13. A computer-readable medium containing a program according to
claim 8 wherein: the program further comprises application software
to apply a semantic context to the set of resources received from
the first location service; and the utilizing software includes
selection software to select the first resource instance based on
the semantic context applied to the set of resources.
14. An apparatus for using a first location service to locate a
resource, the apparatus comprising: a network including at least
two computers; a distributed file system distributed across the
network; a resource accessible via the network; a management
context superimposed on a subset of the network, the management
context including a set of nesting containers containing the
resource; a plurality of location services coupled to the network,
the plurality of location services including at least the first
location service, each location service in the plurality of
location services capable of operating independently of the others;
query means for querying the first location service for the
location of instances of the resource; and utilization means for
utilizing a resource instance returned from the first location
service in response to the query.
15. An apparatus according to claim 14 wherein the query means
includes: sending software to send a message to the first location
service; and receiving software to receive a message from the first
location service.
16. An apparatus according to claim 14 wherein the utilization
means includes software to remotely access the resource instance
returned from the first location service.
17. An apparatus according to claim 14 further: comprising a first
semantic context prioritizing the plurality of location services
for query.
18. An apparatus according to claim 14 further comprising a second
semantic context prioritizing a plurality of resource instances
returned from the first location service.
19. An apparatus according to claim 14 wherein: the resource
includes at least one hardware peripheral connected to the network;
and the first location service returns a location for each hardware
peripheral resource.
20. An apparatus according to claim 14 wherein: the resource
includes at least one instance of a software element on the
distributed file system; and the first location service returns a
location for each instance of the software element resource.
21. A method according to claim 1, wherein: the method further
comprises determining a Global Unique Identifier (GUID) for the
resource using the management context and the name for the
resource; and querying the first location service includes querying
the first location service for the locations of the resource
instances using the GUID for the resource.
22. A computer-readable medium containing a program according to
claim 8, wherein: the program further includes determination
software to determine a GUID for the resource using the management
context and the name for the resource; and the querying software
includes querying software to query the first location service for
the locations of the resource instances using the GUID for the
resource.
Description
FIELD OF THE INVENTION
This invention pertains to distributed file systems, and more
particularly to locating instances of a selected volume over the
distributed file system.
BACKGROUND OF THE INVENTION
In conventional file systems, a client on a workstation can access
a specific named volume on a specific named server. As the volume
name is human-readable, there could be other volumes in the network
with the same name on different servers, but these bear no relation
to any other volumes with the same name. Products exist to
replicate data between volumes on different servers, but the client
is still required to specify which instance of a replicated volume
it wants to access.
A Distributed File System (DFS) eliminates the strong tie between a
file and the server on which it resides. With DFS, volumes still
exist, but they can move between servers or have multiple instances
that exist on different servers. The client specifies only the DFS
volume name or its Global Unique Identifier (GUID) when accessing
files. The advantages of DFS can be generalized to any kind of
resource that can be distributed (e.g., printers, scanners, etc.).
But without some mechanism to assist the client in finding to which
physical server or servers the resource is attached, DFS is of
limited value.
There have been several prior attempts to implement distributed
file systems. The National Software Works (NSW), implemented by the
Advanced Research Projects Agency (ARPA), included a single global
volume distributed across multiple physical machines for file
storage, a solution that did not scale well. Cronus, a distributed
operating system research project undertaken by Bolt Beranek and
Newman (BBN) under contract to the Rome Air Development Center
(RADC), used a (statistically) unique name for the object as a clue
to its location. But if the object was not known by that host
(perhaps because it had been moved), the object would have to be
located by broadcasting a message on the network. This approach did
not scale well, and broadcasting messages can be difficult in any
event. The AFS-3 file system by Transarc Corporation included a
single back-end database implementation with a well-known name for
the volume location servers. This approach is difficult to
generalize, and has a single point of failure (the database).
Microsoft used reparse points, which contain the full list of hosts
where the volume instances can be found. But if a volume moves or a
new instance is added, the reparse points must be modified, which
is a difficult task.
Accordingly, a need remains for a mechanism that allows a client to
locate instances of a resource given the resource's naming
information that is easily scalable, includes redundancies for
continued performance, and is easily updated as volumes are added
to or removed from the DFS.
SUMMARY OF THE INVENTION
To locate an instance of a resource, the management context for the
resource name is consulted. The management context identifies a set
of location services that can locate instances of the resource.
Each location service knows where all resource instances within its
management context are located. One of the location services is
then queried for the location of resource instances. The location
service returns the location of all known instances of any given
resource under the location service's control. One resource
instance is selected. The selected resource instance can be
contacted and utilized.
If either the location service or the selected resource instance
cannot be contacted, alternative location services or resource
instances can be used. As each location service knows of all
instances of resources in the location service's scope, and the
data and metadata of each resource instance are substitutable for
any other resource instance, the techniques for selecting the
location service or resource instance from the given sets are not
significant.
The foregoing and other features, objects, and advantages of the
invention will become more readily apparent from the following
detailed description, which proceeds with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A shows a computer system on which the invention can
operate.
FIG. 1B shows the computer system of FIG. 1A connected to a
network.
FIG. 2 shows a number of volumes in the Distributed File System for
the network of FIG. 1B according to the preferred embodiment.
FIG. 3 is a tree structure showing the logical structure for the
volumes of the Distributed File System in FIG. 2 according to the
preferred embodiment.
FIG. 4 shows how the tree of FIG. 3 is sub-divided into sets of
nodes supported by different volume location services.
FIG. 5 shows how the tree of FIG. 3 is sub-divided into set of
nodes for which different semantic contexts are to be applied to
the network of FIG. 1B.
FIG. 6 shows how the volume location service can be used to locate
an instance of the desired resource over the network of FIG.
1B.
FIG. 7 is a flowchart showing how to use the volume location
service to locate a volume according to the preferred
embodiment.
FIGS. 8A-8B show the effect of applying a semantic context to the
network of FIG. 1B.
FIG. 9 is a flowchart showing how to apply a semantic context in
selecting a volume location service according to the preferred
embodiment.
FIG. 10 is a flowchart showing how to apply a semantic context in
selecting a resource instance according to the preferred
embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1A shows a computer system 105 on which the volume location
service of the invention can operate. Computer system 105
conventionally includes a computer 110, a monitor 115, a keyboard
120, and a mouse 125. Optional equipment not shown in FIG. 1A can
include a printer and other input/output devices. Also not shown in
FIG. 1A are the internal components of computer system 105: e.g., a
central processing unit, memory, file system, etc.
Computer system 105 further includes a location service unit 130
and a resource utilization unit 135. Location service unit 130 is
responsible for finding a volume location service and querying that
service for an instance of the resource. (Although the description
of the preferred embodiment is directed toward locating volumes on
a Distributed File System (DFS), a person skilled in the art will
recognize that the method can be generalized to locating an
instance of any type of resource that can be distributed over the
network.) To that end, location service unit 130 includes two
sub-units: finding sub-unit 130A and querying sub-unit 130B.
Resource utilization unit 135 is responsible for receiving a
pointer to a resource instance and utilizing that resource
instance. To that end, resource utilization unit 135 includes two
sub-units: receiving sub-unit 135A and utilization sub-unit 135B.
In the preferred embodiment, location service unit 130 and resource
utilization unit 135 and their sub-units are implemented in
software.
FIG. 1B shows computer system 105 connected over a network
connection 150 to a network 155. By using network 155, resources,
e.g., computers and their associated software facilities,
peripherals, and data files, such as resources 160A, 160B, and
160C, are accessible. The DFS allows volumes that are physically
located on remote servers to be viewed as part of the local
hierarchy. A special type of link object called a junction allows
references to the roots of other volumes, and ties the many volumes
into a single hierarchy. Thus, rather than thinking about volumes
and their physical connections to networks, it is preferable to
view volumes as containers for their contents.
FIG. 2 shows a number of volumes in a DFS. Because volumes 205-209
can be located anywhere on the DFS, no particular organization is
imposed on the volumes. Each volume can include objects 210.
Objects 210 can include references to more volumes, file objects,
or other resources. One particular type of object 210 is a junction
215. Junctions 215 organize the volumes into a hierarchy,
represented by lines 220.
FIG. 3 shows another way of viewing the logical structure of the
volumes of FIG. 2. In FIG. 3, the logical structure is shown as a
tree. The tree includes nodes 305-309, corresponding to volumes
205-209. Junctions are represented in FIG. 3 by lines 310 that
hierarchically organize the nodes 305-309 into a tree. Each node
has a Globally Unique Identifier (GUID) for a name. The assignment
of GUIDs to nodes is known in the art. GUIDs are generally stored
as 128-bit strings, and are statistically unique. Because GUIDs are
generally not very meaningful to humans, a person skilled in the
art will recognize that each node can have a second name. This
second name will generally be human-readable, but is not guaranteed
to be statistically unique. The human-readable name can also change
over time without affecting the volume location service, which
relies on the GUID of the volume being sought. Each node 305-309
can also include objects 210.
At the top of the tree is a root directory 305. Root directory 305
includes all other nodes, either directly or indirectly. There can
only be one root directory. Root directory 305 is the outermost
container in the network.
Each node 305-309 can include an attribute tab, such as attribute
tabs 315 and 318. (Attribute tabs 315 and 318 were not shown in
FIG. 2 for simplicity.) Attribute tabs 315 and 318 store properties
for the nodes to which they are attached (in FIG. 3, nodes 305 and
308 respectively). Attribute tabs 315 and 318 can be stored as data
fields in the directory system, although a person skilled in the
art will recognize that other techniques can be used to store
attribute tabs. Each attribute tab 315, 318 gives the location of
the volume location services for the node in the tree to which the
attribute tab 315, 318 is attached, as shown in FIG. 6. The volume
location services identified by attribute tab 315, 318 also service
every node in the tree below the node in the tree to which the
attribute tab 315, 318 is attached and that is not covered by
another attribute tab. For example, because node 308 includes
attribute tab 318, the volume location services identified by
attribute tab 318 service nodes 308 and 309. Attribute tab 315,
therefore, does not service nodes 308 or 309. FIG. 4 shows
graphically which nodes in the tree are serviced by which attribute
tabs. Attribute tab 315 services nodes 305, 306, and 307 (shown by
grouping 405); attribute tab 318 services nodes 308 and 309 (shown
by grouping 410). Because each node in the tree must be serviced by
a volume location service, root node 305 must include attribute tab
315; an attribute tab is optional for all other nodes in the tree.
How attribute tabs are used to locate a resource instance is
discussed below with reference to FIG. 6.
The hierarchy of the DFS establishes a management context for each
object in the tree. The "management context" is the set of nesting
containers containing the object of interest. So, for example, the
management context for object 313 includes "Root
Directory/Sub-Directory 3." In general, clients will know the
human-readable name but not the GUID. Using the management context,
objects with the same human-readable name but different management
contexts can be distinguished, and the correct GUID obtained from
the DFS for use with the volume location service.
Returning to FIG. 3, each node in the tree can also optionally
include a semantic context, such as semantic contexts 320 and 321.
(Semantic contexts 320 and 321 were not shown in FIG. 2 for
simplicity.) Semantic contexts 320 and 321 for the node specifies
how the network is to be viewed in using the network. (The term
"semantic context" in this connection is not to be confused with
the term "management context," discussed above.) Semantic contexts
320 and 321 can be stored as data fields in the file system,
although a person skilled in the art will recognize that other
techniques can be used to store semantic contexts. As with
attribute tabs, the semantic context to be applied to the network
is determined by locating the lowest level container containing the
desired object with an attached semantic context. For example, node
305 has attached semantic context 320, and node 306 has attached
semantic context 321. FIG. 5 shows graphically which nodes in the
tree are serviced by which semantic contexts. Semantic context 320
services nodes 305, 307, 308, and 309 (shown by grouping 505);
semantic context 321 services node 306 (shown by grouping 510).
Semantic context will be discussed further below with reference to
FIGS. 8A-8B, 9, and 10.
A person skilled in the art will recognize that, although the above
discussion talks about locating volumes, volume location services
can locate other types of resources. For example, printers are a
type of resource that can be distributed across a network. Network
administrators can add and remove printers at any time without
informing users of the change in available printers. When a
printout is needed, the first step is determining the best printer
for the job. Volume location services can locate printer resources
just as easily as they can locate volumes. Of course, resources
such as printers may not be identical. For example, printing
black-and-white text on color printers is generally more expensive
than printing black-and-white text on a black-and-white printer,
but it is possible. Thus, the volume location service for physical
resources locates the resources themselves, not instances of the
resource, as with electronic resources (where every copy is
absolutely identical).
FIG. 6 shows how a client can use the volume location service to
locate an instance of the desired resource. In FIG. 6, the desired
resource is the object 313. Because object 313 is contained in node
308 and node 308 has an attached attribute tab 318, the volume
location service pointed to by attribute tab 318 is used. Attribute
tab 318 points to volume location services 618A-618D. In FIG. 6
there are four copies of the volume location services; however, a
person skilled in the art will recognize that there can be more or
fewer volume location services. Assume that the client uses volume
location service 2 (618B). The client provides volume location
service 2 (618B) with the GUID of object 313. As discussed above,
the GUID can be obtained from the DFS if the management context is
known. Volume location service 2 (618B) then accesses a distributed
database to determine where instances of the desired object are
located. In FIG. 6, volume location service 2 (618B) would inform
the client that instances 620A-C of the desired object can be found
in containers 625A-C. The client can then select which instance of
the desired object it wishes to access. In FIG. 6 there are three
instances of the resource; however, a person skilled in the art
will recognize that there can be more or fewer resource
instances.
FIG. 7 shows a flowchart of the steps a client takes to use the
volume location service. At step 705, the management context
containing the volume of interest is determined. Then, at step 710,
the GUID of the volume of interest is determined. As discussed
above, given the volume's human-readable name and its management
context, the GUID is easily determined. A person skilled in the art
will also recognize that the steps of determining the management
context for a volume of interest and the GUID of the volume of
interest are completely separate. Therefore, the order of the steps
can be interchanged. At step 715, the volume location service(s)
for the management context are determined. At step 720, if more
than one volume location service is available, one of the available
volume location services is selected. At step 725, the selected
volume location service is queried to locate an instance of the
desired volume. A person skilled in the art will recognize that, if
the selected volume location service cannot be accessed, steps 720
and 725 can be repeated to select an alternate volume location
service. Finally, at step 730, the located instance of the desired
volume is accessed. If more than one instance of the desired volume
is located by the volume location service, the client can access
any one of the instances.
At this point, a person skilled in the art will recognize that, by
taking advantage of the volume location service, administration of
the resource instances can be done entirely behind the scenes,
without the assistance of a system administrator. As resource
instances are added or removed, the DFS can use the volume name and
management context to automatically inform all volume location
services of the changes. The system administrator does not need to
deal with the particulars of updating the volume location service.
In the preferred embodiment, this is even further generalized: the
DFS only needs to inform one volume location service of the change.
The informed volume location service then passes the update to the
other volume location services within the management context for
the volume.
FIGS. 8A-8B show the effect of applying a semantic context to the
network. As a reminder, a semantic context determines how the
network should be viewed while locating a resource instance. In
FIG. 8A, the volume location service has identified three instances
160A-C of the resource desired by the client. Instances 160A-C and
computer 105 are connected by network 155, which includes, among
other links, lines 805, 810, and 815. At this point, before a
semantic context is applied, based solely on the number of hops
required to reach the resource instance, resource instance 160B is
the closest resource instance to computer 105. For example, if
the-resource the client desires is a printer, resource 160B may be
the printer physically closest to the client. (As discussed above,
with physical resources, the volume location service locates the
resource itself, and not an instance of the resource.)
In FIG. 8B, a semantic context has been applied to network 155. The
semantic context specifies a specific policy for resource instance
access and usage. For example, continuing with the example of the
desired resource being a printer, the semantic context has
specified that resource 160A is not available to the client.
Perhaps the printer is reserved for certain users, or is in a
secure location inaccessible to the client. The specific reason is
not important. Applying the semantic context effectively "cuts"
link 805 (e.g., by making the cost of using link 805 infinite), and
this denies the client access to resource 160A.
The semantic context can also change the relative priority of the
resource instances. Continuing with the example of the desired
resource being a printer, resource 160B may be a color printer,
whereas resource 160C is a black-and-white printer. If the client
only needs to print text, the semantic context can increase the
cost of selecting resource 160B, "encouraging" the use of resource
160C.
Although the discussion of FIGS. 8A-8B focused on applying a
semantic context to resource instance selection, semantic contexts
can also be applied in selecting volume location services. Thus, in
locating a resource instance, two semantic contexts may be applied:
one to select the volume location service, and one to select the
resource instance.
As discussed above with reference to FIG. 3, semantic contexts can
be attached to nodes in the tree. This allows for semantic contexts
to be inherited and automatically applied. Alternatively, semantic
contexts can be applied specifically to individual network uses
(for example, by a system administrator).
FIG. 9 shows the steps taken to apply a semantic context in
selecting a volume location service. At step 905 the semantic
context for the volume of interest is located. At step 910, the
network is context-switched based on the located semantic context.
Finally, at step 915, a volume location service is selected based
on the semantic context applied to the network.
FIG. 10 shows the steps taken to apply a semantic context in
selecting a resource instance. At step 1005, the semantic context
for the resource name is located. At step 1010, the network is
context-switched based on the located semantic context. Finally, at
step 1015, a resource instance is selected based on the semantic
context applied to the network.
The volume location service provides advantages over alternative
solutions to distributed file systems. The National Software Works
(NSW) project was an early distributed operating system research
project initiated in 1974 by ARPA. NSW included a Resource Catalog
that implemented a global symbolic name space for objects,
including files. Files and other objects could be located on
various machines in the ARPANET, running various operating systems.
File lookup was handled by a component called the Works Manager.
The Works Manager contained a copy of the entire Resource Catalog.
A component that wished to access a file sent a message to the
Works Manager which then looked up the file and returned a list of
descriptors for all the available physical images of the file. The
requesting component would then choose an available image and copy
it to its local file system by communicating with a File Package
component on the machine that stored the image. So in effect, NSW
provided one single global volume distributed across multiple
physical machines for file storage. The centralized management
nature of the Works manager (there were two Works managers per NSW
system--basically a main and a backup) does not scale well, and
leaves the system susceptible to communication outages--if the link
to the Works Manager is down, the system becomes unusable. In
contrast, the volume location service is easily scalable, as each
volume location service is only required to support a portion of
the DFS. Further, introducing redundancy into the volume location
service is a simple procedure.
Cronus was a distributed operation system research project
undertaken by Bolt Beranek and Newman (BBN) in the early 1980s
under contract to the Rome Air Development Center (RADC). Cronus
was an object-oriented system, and files were one type of object
that could exist in the system. There was a directory, but it
contained only file objects. Objects were identified by a 96-bit
unique identifier. 16 bits of this identifier contained a host
address, which was used as a hint as to the object's location. All
object access occurred though a component known as the Operation
Switch. When a program performed an operation on an object, the
Operation Switch on that machine would attempt to access the object
on the host identified in the unique identifier by contacting the
specified machine's operation switch. If the object was not known
by that host (perhaps because it had been moved), the Operation
Switch would broadcast a Locate message on the network. This
message would be received by all other Operation Switches and they
would determine whether the specified object was hosted on their
machine and if so, send a response to the originating Operation
Switch. This solution does not scale well and does not work well in
a WAN environment, where it is usually difficult to send broadcast
messages. In contrast, the volume location service does not require
any broadcast messages to be sent: clients communicate directly
with the volume location service and the desired resource instance.
Further, as discussed above, the volume location service is easily
scalable.
The AFS-3 file system from Transarc Corporation had a volume
location service that is in many ways similar to the volume
location service described herein. However, there are significant
differences. AFS-3 provided a single back-end database
implementation. It did not have a directory service available to
tie the service into. AFS-3 used a configuration file with a
well-known name to find volume location servers. In contrast, the
volume location service described herein is tied into an available
directory service, without relying on a single back-end database.
Further, the names of the servers storing the volume location
services are not fixed, generalizing the solution.
Microsoft Corporation describes its distributed file system as
using something called reparse points, which are objects in the
existing file system. These reparse points contain the full list of
hosts where the volume instances can be found. Thus, a distributed
file system volume is "mounted" in an existing file system
directory, and if a volume moves or a new instance is added, it
will be necessary to modify the reparse point itself. In the
general case, this would be a major undertaking, as there could be
many instances of different reparse points scattered throughout the
entire file system. Microsoft solves this problem by only allowing
reparse points to exist at one point in the directory tree. The use
of junctions is more general, as junctions can appear as
directories anywhere in the file system and allow the physical
location information to be updated in a single place for all volume
references.
Having illustrated and described the principles of our invention in
a preferred embodiment thereof, it should be readily apparent to
those skilled in the art that the invention can be modified in
arrangement and detail without departing from such principles. We
claim all modifications coming within the spirit and scope of the
accompanying claims.
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